Singapore residents have been urged to stay indoors amid unprecedented levels of air pollution.

The prime minister of Singapore — which prides itself on its clean environment — has warned that the haze could hang over the city state for weeks.

The smog has been blamed on illegal forest fires on the Indonesian island of Sumatra. Companies burn forest to clear space for palm oil plantations, with most smoke haze problems arising during the dry season between June and September.

The Singapore government warned Singapore-linked companies against participating in the burn-offs, with Prime Minister Lee Hsien Loong saying:

"If any Singapore companies are involved, or companies which are present in Singapore are involved, we will take it up with them."

At 1 PM local time in Singapore, the pollution standards index — provided by the National Environment Agency — reached 371, breaking all previous records and passing the "hazardous" mark.

On the country's border with Singapore, however, "hazardous" pollution was recorded in at least one district and some 200 schools ordered shut. The Department of Environment has also banned open burning in the three states nearest Sumatra, making it punishable by up to five years in prison.